Home Made Explosives (HME) and Improved Explosive Devices (IED) are weapons of choice for terrorists worldwide and at home. Critical to the safety of our service members and the public are uncovering these HMEs and IEDs before they are employed as well as investigating an incident after it occurs to prevent future occurrences. The ability to detect trace or residue explosive material can make a significant impact in prevention, mitigation and deterrence in the global effort against terrorists.
Traditional explosive detection devices uses instruments such as portable electronic devices, raman spectroscopy, capillary electrophoresis, laser electrospray with mass spectrometry, ion chromatography, and laser induced breakdown spectroscopy. Use of such instruments require specialize equipment and training, a large budget and regular maintenance. While these devices are useful, their costs, training, and limited portability may not be ideal for use by soldiers on the field. Alternatives to instruments for detecting explosives uses colorimetric explosive detection kits which detect the presence of explosives or a key ingredient in the explosive by a visual color change. This is attributed to a unique chemical reaction between the chemical of interest and a reagent. Colorimetric methods rely on several chemical reagents used in a sequence of steps to perform the necessary reaction for a color change to occur. Typically these detection capabilities rely on highly specific reactions to confidently determine the explosive present.
Much effort has been developed in the detection of a number of explosives including those that contain chlorate, hydrogen peroxide, ammonium, urea and nitrates. For instance, potassium salt of chlorate is a strong oxidizer, readily available and easily detonable when mixed with fuel. Numerous examples of HMEs exist that contain potassium chlorate such as Armstrong's mixture, black powder variants, poor man's C-4, and Sprengel explosive. Hydrogen peroxide is a strong liquid oxidizer and a critical ingredient in the preparation of explosive organic peroxides such as triacetone peroxide (TATP), hexamethylenetriperoxidediamine (HMTD), and methyl ethyl ketone peroxide (MEKP). The detection of hydrogen peroxide is a good indicator for the presence of these explosive peroxides. Ammonium and urea nitrates found in fertilizers are also common in HMEs because of its widespread accessibility and ease for concealment as an agricultural use product.
Colorimetric detection of explosives are generally known in the literature. For example, U.S. Pat. No. 7,846,740 titled “Method and Kit for detecting explosive substances containing certain oxidants” describes the blue color formation of chlorate detection when treated with the chemical reagent diphenylamine in the presence of sulfuric acid, dimethyl sulfoxide and ethanol. In this case the oxidation of diphenyl amine produces a colored compound which is presumably the meriquinoidal diphenylbenzidine blue intermediate known from the literature. Other colored reactions occur which have a similar reaction mechanism and collectively these are known as redox reactions. Typically redox reaction with chlorate occur with a number of aromatic amines and aromatic alcohol derivatives.
Still other methods of non-instrumented color detection chemistries of chlorate detection exist. For example, manganous sulphate in concentrated phosphoric acid reacts with chlorate to produce the violet manganic-phosphate ion. A different method for the detection of chlorate uses ammonium thiocyanate catalyzed by iron to produce a yellow color change from the oxidation of thiocyanate; however, both of these methods require heat to facilitate the reaction to occur in a timely manner.
Hydrogen peroxide is also detected by redox chemistry. Probably the most common method for the detection is the iodine-starch test. Other methods include the oxidation of ferrous ions and reaction with xylenol orange, peroxidase enzyme reactions, and a number of metal oxidations that include iron, copper, and vanadium complexes to list a few. The detection of hydrogen peroxide is a suitable indicator for the presence of organic peroxide containing HMEs. Typically this requires a stepwise procedure that begins with the decomposition of the organic peroxide to produce hydrogen peroxide followed by a second reaction step for identification.
Commercially available colorimetric kits for the detection of explosive compounds are available but with varying levels of success. A few such kits are Explosive Testing Kit (ETK) from Lindon Defense, Elite Test Kit from Field Forensics, Inc., and DropEx kits from Mistral Security, Inc. These kits use liquid reagents (sometimes in multi-step sequences) to react with the suspected explosive to perform the necessary chemical reaction for a color change to occur. This approach is sometimes referred as wet chemical analysis.
Current liquid explosive detection kits require the user to premix liquid chemicals using multiple vials of liquids and performing serial analysis using specialized collection equipment typically packaged in a bulky container. The drawbacks associated with liquid explosive detection kits is the inherent risk of liquid reagent spillage. In cases where hazardous chemicals are used (such as strong acids) personal safety equipment such as gloves and glasses are highly recommended. In addition, spillage and unintentional mixing of liquid reagents may lead to unreliable results. All of these limitations to liquid explosive detection kits can be difficult for use on the field or under stressful situations. Thus, a need exist for an easy-to-use portable, safe, fast and reliable colorimetric explosive detection kit.